1. Field of the Invention
The present invention relates generally to a steering system for vehicles. More particularly, the invention relates to a steering system for a vehicle with front and rear wheels, of the type which includes a front wheel steering mechanism for steering the front wheel in accordance with a steering operation of a driver and a rear wheel steering mechanism for steering the rear wheel in accordance with a travelling state of the vehicle.
2. Description of Relevant Art
There have been proposed various steering systems for vehicles with front and rear wheels, such as four-wheeled motor vehicles; an exemplary steering system of such type being disclosed in U.S. Pat. No. 4,412,594. Each such steering system includes a front wheel steering mechanism for steering the front wheels at an angle in accordance with a steering operation of a driver of the vehicle and a rear wheel steering mechanism for steering the rear wheels at an angle in accordance with a travelling state of the vehicle.
By virtue of such a steering system, both the turning characteristics and the steering responsiveness of vehicles can be greatly improved.
Generally, in vehicles of a type having road wheels suspended from a vehicle body through suspensions, respectively, there has been a problem in that, while performing cushioning actions, the suspensions produce steering variations of the wheels, such as a bump steering effect and a rebound steering effect, for example. Such variations result from geometrical characteristics, depending on the construction of the suspensions and corresponding to the cushioning actions, and act as a disturbance to the steering performance of such vehicles.
Particularly, in a popular four-wheeled automobile having a vehicle body with an engine mounted on the front part thereof and thus being lighter in weight at the rear part than at the front part thereof, such variations tend to develop more seriously on the rear wheels than on the front wheels. Thus, when travelling on an expressway with a damaged road surface, for example, the vehicle may suffer from steering variations at the rear wheel side, corresponding to cushioning actions of rear suspensions due to undulations of the road surface, to the point of causing a fish-tailing motion.
In this respect, in a vehicle including a front wheel steering mechanism for steering front wheels at an angle according to a steering operation of a driver, although steering variations developing on the front wheels may be absorbed by the steering operation, it is not easy to compensate for those on the rear wheel side.
Moreover, in a vehicle of the type which has road wheels suspended from a vehicle body through suspensions, there has been another problem in that, when the vehicle is turning, the vehicle body rolls, producing a difference of cushioning action between left and right suspensions corresponding to the roll angle. Corresponding steering or camber variations are thus caused to develop on those wheels suspended by the left and right suspensions, which unfavorably affects steering of such vehicle.
Further, such a rolling motion produces, between tires of the left and right wheels, a difference of the contact load to the ground in accordance with the roll angle, while causing the lateral force characteristic to be different between tires of the front and rear wheels, so that the steering responsiveness changes as a steering wheel is turned.
In this respect, in a vehicle with a front wheel steering mechanism for steering front wheels at an angle according to a steering operation, a driver is expected to cope with such steering variations, as well as the difference in steering responsiveness, through a steering correction by the steering operation to the front wheels. However, considerable steering skill is required to smoothly perform such an operation.
The present invention effectively solves the foregoing conventional problems of a vehicle of the type including a steering system as described hereinabove.